1. Field of the Invention
The present invention relates to synthetic fiber extrusion packs and polymer distribution plates for fiber extrusion packs.
2. Description of the Related Art
Fabrics made from melt-spinnable polymers are commonly produced using spunbond processes. The term spunbond refers to a process of forming a non-woven fabric or web from an array of thin, melt-spun polymeric fibers or filaments produced by extruding molten polymer from orifices. In a typical, large production spunbond machine, polymer pellets placed in a hopper are fed to a screw extruder which melts the polymer. The molten polymer then flows through a heated pipe to a spin beam. The spin beam is essentially a pressure vessel which uses boiling Dowtherm or a similar liquid/vapor material to provide uniform heat to a spin pack contained therein. The spin beam also contains one or more metering pumps which receive the molten polymer and control the polymer flow rate through the spin pack.
The spin pack can be constructed from a series of thin flat, generally rectangular plates into which small channels and holes are etched or machined. When layered in a stack and bolted together to form the spin pack, the plates form a distribution network for distributing the molten polymer to the spinneret at the lowermost portion of the spin pack. Such a distribution network is generally required only where bi-component or multi-component fibers are to be produced. Consequently, it will be appreciated that spunbond packs with multiple plates that form distribution networks as described herein, generally have applicability in the formation of plural-component fibers.
The spinneret contains an array of spinning orifices, typically from 1000 to 5000 per meter of length of the spinneret, arranged in a substantially horizontal, rectangular array, with each orifice extruding an individual polymer stream and being shaped to form fibers of a particular cross section and size. Below the spinneret, the array of extruded polymer streams forms a vertically oriented curtain of downwardly moving strands that are drawn as they cool and then further processed to form fabric (e.g., additional in-line processing can be performed to produce a non-woven web or fabric, or the fibers can be wound on a roll for later processing).
A plan view of a representative distribution plate 10 of a conventional spunbond pack is shown in FIG. 1. For convenience, the polymer distribution features that would be present on such a plate are omitted from the drawing. Further, the slots and holes shown in FIG. 1 are not necessarily drawn to scale. In the example shown, plate 10 is approximately twelve feet in length in the cross direction. This dimension essentially dictates the maximum width of the extruded fiber curtain that can be produced and the corresponding maximum width of a non-woven web formed by laying the extruded fibers on a moving belt. In the machine direction, such plates are typically about eight to ten inches wide, although plates as wide as about sixteen inches have been used.
The spunbond pack is essentially a layered structure formed by overlaying several distribution plates on each other in a stack, with each layer being formed by a single plate, such that the features of adjacent plates are aligned to form the channels and through holes that constitute the polymer distribution network. The plates are aligned using a dowel pin which is inserted into a central dowel pin hole 12 located near the center of the plate in the cross direction and toward one edge of the plate in the machine direction, as shown in FIG. 1. The central dowel pin hole 12 is substantially circular and closely matches the cross-sectional shape and dimensions of the central dowel pin, such that the central dowel pin controls the end-to-end alignment of the plates (i.e., the central dowel pin properly aligns the plates in the cross direction) when the dowel pin extends through the stack.
Two additional, substantially slot-shaped dowel pin holes 14 are respectively located near the two cross-directional ends of plate 10 toward one edge of the plate in the machine direction. The slots are aligned in the cross direction, permitting a small degree of play in this direction (i.e., the dowel pins inserted in the outer slot-shaped dowel pin holes do not control the end-to-end alignment in the manner of the central dowel pin). Two corresponding dowel pins are inserted in these outer slot-shaped dowel pin holes 14 and extend through the stack of plates to ensure proper alignment in the machine direction and effectively prevent rotational misalignment between plates. The remaining circular holes shown in FIG. 1 are bolt holes through which bolts are inserted to fasten the plates together to form the fiber extrusion pack.
In spunbond packs over one meter long, there is a limit to design density due to the inherent alignment inaccuracy at the ends of the pack. For example, using certain plate manufacturing processes, any feature on the plate may have a long-term positional accuracy on the order of 0.002 inch per foot from the central dowel pin. With a spin pack constructed from twelve foot long plates, such as that shown in FIG. 1, the ends of the plate are six feet from the central dowel pin. Consequently, a twelve foot long pack would have a plate-to-plate alignment accuracy of approximately 0.012 inches (6 ft.×0.002 inches/ft.). These tolerances significantly limit the density of the pattern design in the distribution plates and thus the spin hole density, since the features on the plates must be large enough to accommodate potential misalignments of this size. While plate-to-plate alignment accuracy can be improved somewhat by reducing manufacturing tolerances, there are inherent limitations to manufacturing accuracies, and greater accuracy requirements generally increase manufacturing costs.
Accordingly, it would be desirable to reduce the plate-to-plate misalignments between adjacent layers of fiber extrusion packs to permit higher pattern design density and smaller features (e.g., holes and channels) in the plates of fiber extrusion packs.